1. Made VP8_SET_REFERENCE vpx_codec_control API work in VP9 based
on Ryan Overbeck's patch. (Thanks.)
2. Added vp9cx_set_ref example, which demonstrated how to set
VP8_SET_REFERENCE vpx_codec_control in encoder and decoder. If
we only set_reference in the encoder, the encoder/decoder
mismatch would be observed.
3. Also updated test/cx_set_ref.sh.

Change-Id: I433a6220616947ca8c73e65a5fb3694751ea84b6